Jet mills are operated by compressed air, inert gas or steam as a propellant. For the sake of simplicity, it is always compressed air which is mentioned as a propellant (by way of example) in the following description. When charging product by means of injectors into jet mills and, in particular, into spiral jet mills, blow-back may occur, for example, when the injector collecting nozzle (Venturi meter) builds up an insufficient counterpressure compared to the mill internal pressure or is caked with product. The mill internal pressure is dependent upon the milling chamber hold-up. During no-load operation, a high internal pressure has to be overcome. In many cases, this calls for collecting nozzles with a small mixing stage diameter. The mill internal pressure drops dramatically as soon as there is product in the milling chamber. In said state, it would be perfectly possible to select an injector collecting nozzle with a larger diameter. What actually often happens in practice is that a jet mill is operated with collecting nozzles which are as large as possible in order to facilitate the charging of products which are difficult to fluidize or which bake on. In said case, a continuous product feed has to be ensured because otherwise a return flow into the injector and into the product charging apparatus arises (blow-back). This calls for careful supervision by the attendant. Manufacturers of jet mills and, in particular, of spiral jet mills have not, up till now, offered automated supervision to prevent blow-back of the charged product at upstream injector metering devices.